This invention is concerned with high energy density pumping technology, specifically electro-osmotic pumps, and with the use of such pumps to implement nastic actuators.
In electrochemistry, physics and vascular plant biology, electro-osmosis is defined as the motion of ions in a solvent environment, under the influence of an applied electric field, through very narrow channels, as in a membrane or other porous structure, generally along a charged surface or through a non-macroporous material which has ionic sites and allows for the inflow or outflow of water (sometimes referred to as “chemical porosity”). Electro-osmotic flow can occur in natural unfiltered water, as well as buffered solutions.
The cause of electro-osmotic flow is an electrical double layer that forms at the stationary/solution interface. In capillary electrophoresis, the narrow channels are made up of silica, and silanol groups form the inner surface of the capillary column. These silanol groups are ionized when the pH is higher than 3. Thus, the inner surface of the channel is negatively charged. In solutions containing ions, the cations will migrate to the negatively charged wall, causing the formation of the electric double layer.
When an electrical potential is applied to the column, with an anode at one end of the column and a cathode at the other, the cations will migrate towards the cathode. Since these cations are solvated and clustered at the walls of the channel, they drag the rest of the solution with them, even the anions. This results in an electro-osmotic flow. Any combination of an electrolyte (a fluid containing dissolved ions) and an insulating solid will generate electro-osmotic flow.
Interest in electro-osmotic flow has increased with the realization that this mechanism provides a very efficient way to generate fluid flows in microfluidic devices, with some electro-osmotic pumps capable of generating flow rates as large as a few milliliters per minute and others producing pressures as large as hundreds of atmospheres.
One application for which electro-osmotic pumps are particularly suited is nastic actuation. In biological systems, nastic movements, usually associated with plants, are rapid, reversible responses to changes in the environment, such as temperature, humidity or light irradiance. These movements are caused by changes in the water pressure inside the plant and can result in very large changes in shape. An example of such a response is the opening and closing of flowers (photonastic response).
Actuators are devices that transform an input of energy into mechanical work. Most actuators use electricity for the driving energy, but nastic actuators function due to an increase in internal osmotic pressure. Each actuator can be very small, allowing a structure to be packed with them, the individual work of each actuator adding up to result in a wide range of movement, enabling net shape change.
Research in nastic materials is developing synthetic intelligent materials that utilize internal pressure changes to effect controlled shape changes, for a new generation of biologically inspired engineering systems. Researchers are working on using nastic materials, for example, to create a morphing aircraft wing that could dynamically change shape and control surfaces during flight. Airplane wings imbued with such ability might, for example, self-trim as they sensed the plane's speed, or prepare for an imminent landing by cupping the air as a bird's wing does. Actuation mechanisms generate axial extension/contraction and bending by incorporating active transport control to actuate nastic structures.
Active actuation materials can generate large strains while carrying significant structural loads. Nastic structures capable of shape change can be configured with arrays of miniature hydraulic actuators, with distributed fluid, active transport control mechanisms. A distributed hydraulic actuator could potentially match the energy density of traditional hydraulic systems. Without a centralized hydraulic pump, such a hydraulic system is operated by distributed active control based on electro-osmotic transport of a working fluid.
These devices can be synergistically integrated with mechanisms employing superabsorbent polymers (SAP) that can absorb and retain extremely large amounts of a liquid relative to their own mass. SAPs, or hydrogels, are cross-linked networks of flexible polymer chains carrying ionic functional groups. These fixed ionic groups attract counter ions and gradients of ionic concentrations give rise to osmotic pressure that drives the swelling of an SAP in an aqueous environment. The total absorbency and swelling capacity are controlled by the type and degree of cross-linking to the polymer. The absorbability of the SAP can be electrochemically controlled to regulate swelling, shrinkage and pressure, thereby altering the cell size of a nastic structure.
Electro-osmotic pumps in the prior art utilize a disk-shaped electric double layer (EDL) material to create the pumping action. Such a shape, however, has a limited surface area and thus cannot sustain a high back pressure, because of its flat geometry. The limited surface area consequently limits the pumping flow rate that can be sustained by such an electro-osmotic pump.
Consequently, a need has developed in the art for an electro-osmotic pump with a larger active pumping surface area and a design that is mechanically able to withstand high pressures. Existing pump designs can provide either a relatively high pressure or a relatively high flow rate, but not both. It would be desirable to provide an electro-osmotic pump that can provide both of these features, i.e., a pump that will operate with a high energy density.